Method of separating chlorine and sulfur dioxide from mixtures thereof



Patented May 22, 1951 METHOD OF SEPARATING .QHLORINE AND SULFUR DIOXIDEFROM MIXTURES THEREOF Arthur W. Hixson, New York, N.

Leonia, N. J and Ralph Miller, Y., .assignors to The ChemicalFoundation, Incorporated, a membership corporation of New York NoDrawing. ApplicationOctober 1, 1946, Serial No. 700,378

11 Claims. 1

This invention relates to a method of separating .chlorine and sulphurdioxide from mixtures thereof.

This application is a continuation in part of application, Serial No.400,662, filed July 1, 1941, now forfeited.

It has been found, as is more particularly explained in copendingapplication, Serial No. 263,190, filed March 21, 1939, now Patent No.2,441,550, that it is possible to produce sodium sulphate and chlorinefrom sodium chloride by ellective methods. In one general method liquidsulphur triox-ide was contacted with sodium chloride to form sodiumchlorosulfonate which was thermally decomposed to form sodium sulphate,-

chlorine and sulphur dioxide; the sulphur dioxide and chlorine wereseparated, the separated sulphur dioxide oxidized to the trioxide andemployed for reacting with further amounts or sodium chloride.

In another method chlorosulfonic acid was reacted with sodium chlorideto form sodium chlorosulfonate and gaseous hydrogen chloride. Thechlorosulfonate was thermally decomposed to form sodium sulphate,sulphur dioxide and chlorine; the sulphur dioxide and chlorine wereseparated, the separated sulphur dioxide was oxidized and reacted withthe hydrogen chloride to form the chlorosulfonic acid which was employedfor further reaction with salt.

.In addition to the above outlined two step methods the inventiondescribed in the earlier application comprehended a direct conversion.In this operation sulphur trioxide was directly re acted with sodiumchloride under such elevated temperature conditions as to directly formsodium sulphate and a mixture of sulphur dioxide and chlorine insubstantially equimolecular proportions. The sulphur dioxide wasseparated from the chlorine as by preferential liquefaction, oxidized tothe trioxide and, absorbed by oleum. The oleum was then heated to evolvesulphur trioxide and this, undiluted with oxygen, was used to react withthe salt.

A major important step in each of these methods is the separation of thegaseous reaction products, sulphur dioxide and chlorine. The methods ofseparation described in the earlier application while efiicient requiredeither low temperature or high pressure, for liquefaction, whichnecessitated careful control and relatively expensive equipment.

It has now been found that the described separation may be very simplyand economically effectuated. Under the new method the separation may beeiiected under wide permissibe variations of temperature and pressurethus permitting its adaptation to optimum chosen conditions. As will beseen the new method also insures a rapid and complete separation andwithout the consumption of chemical reagents.

The invention broadly comprehends the concept of selectively reactingsulphur dioxide with a material with which it forms an addition compoundand from which it may be evolved or liberated by elevation oftemperature and which is inert towards chlorine.

As is known, chlorine is more volatile than sulphur dioxide. The presentinvention advantageously utilizes this difference in volatility. Thepreferred selective reactant for the new method is anhydrous aluminumchloride. This reacts with sulphur dioxide to form an addition compoundwhich may be solid or liquid depending upon the temperature. Underordinary conditions the addition compound is a viscous liquid at C. Whenthe temperature is reduced the solution becomes more and more viscousand con.

versely when the temperature is raised it becomes morefluid.

A favorable feature of the compound is that the vapor pressure ofsulphur dioxide above it is small at ordinary temperatures, such vaporpressure being about equivalent to atmospheric pressure at about C.Another important characteristic of aluminum chloride is that it doesnot react with chlorine so that the action with sulphur dioxide is trulyselective.

The present invention is based on these selective features. Whenaluminum chloride is contacted with a gaseous mixture containingchlorine and sulphur dioxide at a temperature of somewhat below 140 C.the chloride reacts selectively wtih sulphur dioxide to form theaddition compound. In practical effect this decreases the vapor pressureof the sulphur dioxide without affecting the vapor pressure of thechlorine. It will be obvious that if elevated pressures are desired tobe employed the temperature range may be extended.

It will be appreciated that with the described specificity of action ofthe aluminum chloride the process may be carried out in a variety ofmethods. In one illustrative operation the absorbent material may beutilized in two or more towers through which the gases to be treated mayalternately be passed. The gaseous mixture to be separated may be passedthrough one tower packed with aluminum chloride. This tower preferablyis provided with suitable heating and cooling means. The gas is allowedto flow through at such a rate that all the sulphur dioxide in theenterin mixture reacts with the chloride. The chlorine, free fromsulphur dioxide, which is withdrawn from the tower is passed to areceiver.

When the aluminum chloride in the one tower has become exhausted bycomplete reaction, the flow of entering gas is then diverted to a secondtower of the battery. Pure sulphur dioxide may then be passed throughthe first tower to sweep out any residual chlorine and this gas mixturemay be passed through the second tower for fractionation therein.

After the first tower has been cleared of chlorine the flow of sulphurdioxide therethrough is stopped and the addition compound is treated insitu to regenerate aluminum chloride. This may be done by heating themass to a temperature above about 140 C. to drive off the sulphurdioxide. By suitable connections this evolved sulphur dioxide may be ledto a gas holder and may subsequently be oxidized to sulphur trioxidewhich may be reacted with sodium chloride by any of the several methodsdescribed in the earlier application. After heat treatment of theaddition compound and evolution of its combined sulphur dioxide, thefirst tower may be put back in the circuit. By thus alternating theentering gas stream between two or more towers a continuous operationmay be carried out.

If desired, the spent aluminum chloride may be removed from the towerand passed to a separate unit for regeneration therein. In this event itis desirable to first heat up the addition compound to increase itsfluidity so as to permit gravity or pressure flow from the tower. Itwill also be appreciated that the aluminum chloride may be extenuated toany desired degree by employing it on inert carriers.

It is to be observed that in the regeneration step when the additioncompound is heated to the neighborhood of 140 C., under atmosphericpressure, only about one-half of the sulphur dioxide present is evolved.Thus, after the initial contacting of the aluminum chloride with sulphurdioxide the material which is returned to the absorption portion of thecycle will have the approximate composition (AIC13)2SO2. Upon contactwith sulphur dioxide containing gas at a lower temperature a reactiontakes place which may be indicated as follows: (A1C13)2SO2+SO2 ZAlChSOz.Hence, where aluminum chloride is described herein as the absorbent itis to be understood that such absorbent may vary in composition between(A1Cl3)2SO2 and A1C13SO2.

The process with equal facility may be carried out using aluminumchloride in liquid phase. For this operation the reaction betweensulphur dioxide and the chloride is carried out at the appropriately lowtemperature and/or elevated pressure. It is preferable, other thingsbeing equal, to carry out the operation at elevated pressure for withsuch elevated pressure a higher temperature with consequent improvedfluidity of the addition compound is insured. Operating at higherpressures also permits the ready separation of chlorine by releasing thepressure and removing it from the less volatile addition compound.

As will be appreciated, the novel principles of the invention may beembodied in a number of specifically different methods utilizin a widevariety of apparatus. The process described is me y ill t at ng. a t icas e l e ment of the fundamental principles which constitutes theinvention.

We claim:

1. A method of separating sulphur dioxide and chlorine from a gaseousmixture of these components which comprises, contacting the mixture withsolid aluminum chloride under conditions regulated to efiect theformation of an addition compound of the chloride and sulphur dioxideand recovering chlorine substantially free from sulphur dioxide.

2. A method of separating sulphur dioxide and chlorine from a mixturethereof which comprises, contacting the mixture with anhydrous aluminumchloride under conditions which insure the formation of a solid additioncompound of the chloride and sulphur dioxide, separating chlorine andthe addition compound and heating the addition compound to evolve thecombined sulphur dioxide.

3. A method of separating sulphur dioxide and chlorine from a gaseousmixture thereof which comprises, continuously contacting anhydrousaluminum chloride with a flowing stream of the gaseous mixture topreferentially abstract the sulphur dioxide from the mixture to insurethe formation of a non-gaseous addition compound of sulphur dioxide andthe chloride and continuously removing chlorine substantially free fromsulphur dioxide.

4. A method of separating sulphur dioxide and chlorine from a gaseousmixture thereof which comprises, continuously contacting anhydrousaluminum chloride with a flowing stream of the gaseous mixture at atemperature below C. to preferentially abstract the sulphur dioxide fromthe mixture by insuring the formation of an addition compound of sulphurdioxide and the chloride and continuously removing chlorinesubstantially free from sulphur dioxide.

5. A method of separating sulphur dioxide and chlorine from a gaseousmixture thereof which comprises continuously contacting solid aluminumchloride with a flowing stream of the gaseous mixture to therebydirectly form a non-gaseous addition compound of sulphur dioxide and thechloride, continuously removing chlorine substantially free from sulphurdioxide, thermally decomposing the addition compound to form sulphurdioxide and aluminum chloride [and reemploying the regenerated aluminumchloride in the process.

6. A method of separating sulphur dioxide and chlorine from a liquidmixture thereof which comprises, contacting the mixture with anhydrousaluminum chloride under conditions regulated to effect the formation ofan addition compound of the chloride and sulphur dioxide and separatingchlorine from the said compound.

'7. In the method of producing sodium sulphate and chlorine from sodiumchloride that improvement which comprises, reacting sulphur trioxidewith sodium chloride under condition regulated to form sodium sulphateand a mixture of chlorine and sulphur dioxide in substantiallyequimolecular proportions, removing the gaseous mixture from the solidproducts of the reaction, contacting the gaseous mixture with aluminumchloride which preferentially reacts with the sulphur dioxide to effectthe formation of a non-gaseous addition compound, separating thechlorine from the said compound, thermally decomposing the compound toform sulphur dioxide and regenerated aluminum chloride, mixing therecoveredsulphur dioxide with air and under conditions regulated tooxidize the sulphur dioxide to sulphur trioxide, absorbing the sulphurtrioxide thus formed in sulphuric acid to produce oleum, heating theresulting oleum to recover undiluted sulphur trioxide and reacting theproduced sulphur trioxide with additional sodium chloride in the firstreaction stage for the continued production of sodium sulphate andchlorine.

8. The process of producing chlorine and sodium sulphate comprising,burning sulphur-containing material to form sulphur dioxide; oxidizingthe sulphur dioxide to sulphur trioxide; cooling the sulphur trioxideand absorbing it in oleum; heating the oleum to evolve sulphur trioxide;contacting sulphur trioxide with sodium chloride to form sodiumchlorosulfonate; thermally decomposing the sodium chlorosulfonate toform gaseous chlorine, sulphur dioxide and solid sodium sulphate,separating the solid sodium sulphate from the gaseous mixture,contacting the gaseous mixture wtih anhydrous aluminum chloride underconditions regulated to form an addition compound of sulphur dioxide andthe chloride; separating gaseous chlorine from the solid additioncompound; thermally decomposing the solid addition compound to formsulphur dioxide and regenerated aluminum chloride; oxidizing therecovered sulphur dioxide to sulphur trioxide for recycling in theprocess and reemploying the regenerated aluminum chloride for separationof sulphur dioxide from its mixture with chlorine.

9. A process of removing sulphur dioxide from a gaseous mixture ofsulphur dioxide and chlorine which comprises, contacting said gaseousmixture with a non-gaseous reagent derived from aluminum chloride andsulphur dioxide, the contacting taking place at a temperaturesubstantially below 140 C., whereby a non-gaseous addition compound ofaluminum chloride and sulphur dioxide is formed, separating the gaseouschlorine from said non-gaseous addition compound, heating saidnon-gaseous addition compound to a temperature not substantially lowerthan 140 C., whereby a substantial part of the sulphur dioxide is drivenoff as a gas, separating said gaseous sulphur dioxide from thenorigaseous residue and cooling said separated nongaseous residue to thecontacting temperature whereby said separated non-gaseous residue ismade ready for reuse in the process.

10. The method of separating sulphur dioxide and chlorine from a liquidmixture thereof which comprises, contacting the mixture with ananhydrous reagent derived from aluminum chloride and sulfur dioxide, thecontacting taking place under conditions regulated to effect theformation of an addition compound of aluminum chloride and sulphurdioxide and separating chlorine from the said compound.

11. A method of removing sulfur dioxide from a gaseous mixture of sulfurdioxide and chlorine which comprises, contacting said gaseous mixturewith a non-gaseous reagent, the contacting taking place under conditionsregulated to effect the formation of a non-gaseous addition compound ofaluminum chloride and sulfur dioxide and separating the gaseous chlorinefrom the said non-gaseous addition compound; saidnon-gaseous reagentbeing prepared by heating a non-gaseous addition compound of aluminumchloride and sulfur dioxide to a temperature at which a substantialportion of the sulfur dioxide is driven off as a gas and separating thegaseous sulfur dioxide from the non-gaseous residue, said nongaseousresidue being the aforesaid non-gaseous reagent.

ARTHUR W. I-IIXSON. RALPH MILLER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,375,011 McAdam May 1, 19452,377,138 Farrell et al May 29, 1945 OTHER REFERENCES Thomas AnhydrousAluminum Chloride; Reinhold Publishing Co., N. Y. (1941).

1. A METHOD OF SEPARATING SULPHUR DIOXIDE AND CHLORINE FROM A GASEOUSMIXTURE OF THESE COMPONENTS WHICH COMPRISES, CONTACTING THE MIXTURE WITHSOLID ALUMINUM CHLORIDE UNDER CONDITIONS REGULATED TO EFFECT THEFORMATION OF AN ADDITION COMPOUND OF THE CHLORIDE AND SULPHUR DIOXIDEAND RECOVERING CHLORINE SUBSTANTIALLY FREE FROM SULPHUR DIOXIDE.